Can I dual boot 9 and 10?

[Head_on_a_Stick]

If the hard drive is using a GUID partition table then just delete the swap lines in /etc/fstab for all installed distributions that boot with systemd and let that automount them instead, then you won't have to worry about updating the entries ever again.

[uthappam]

Thank you. This is much simpler than the route I had imagined I would have to take, involving BLKID, UUIDGEN, TUNE2FS then two each of FSTAB and GRUB. But I must search for the meaning of GUID.
[addition] Bad news. The partition table is MSDOS.

[Head_on_a_Stick]

Check your partition table type with

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# parted -l

Look for the "Partition Table" section for your disk, "gpt" means the disk has a GUID partition table.

[p.H]

If the hard drive is using a GUID partition table then just delete the swap lines in /etc/fstab for all installed distributions that boot with systemd and let that automount them instead

The swap is also used for hibernation (suspend-to-disk). This won't fix the delay before "Gave up waiting for suspend/resume device".
Remove or update any reference to the swap in /etc/initramfs-tools/conf.d/resume and rebuild the initramfs with

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update-initramfs -u

the route I had imagined I would have to take, involving BLKID, UUIDGEN, TUNE2FS then two each of FSTAB and GRUB.

You don't need uuidgen. tune2fs is for ext* only, not swap. GRUB does not care about swaps.
You have two routes :
a) update or remove all references to the swap UUID in Stretch in fstab and initramfs.
b) reset the previous UUID with swaplabel and update or remove all references to the swap UUID in Buster.

[uthappam]

Thank you, p.H. I did these things (cannot remember where on the Net I found them): used blkid to identify the swap partition, then put the UUID into /etc/fstab and /etc/initramfs-tools/conf.d/resume. Ran sudo update-initramfs -u. When I rebooted, Debian 9 started up without delay or error messages. I then went into Buster and repeated what I had done in Stretch. After it was asked to update initramfs, it complained that there was "no matching swap device available." Strange, fstab points to /etc/sda5 in both Debians.

[p.H]

You did not have to change anything in Buster, it already referenced the up-to-date UUID.
Please post the output of the following commands in Buster :

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blkid | grep swap
swapon -a
grep swap /etc/fstab
cat /etc/initramfs-tools/conf.d/resume

[uthappam]

Here are the results of running the commands you listed:

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blkid | grep swap
/dev/sda5: UUID="f0bcc9fd-b8f9-445c-aaf1-b4f06cbc20f4" TYPE="swap" PARTUUID="922fc955-05"

swapon -a
swapon: cannot find the device for UUID=16658fa8-8468-4cf1-955a-8bcc2e588741

grep swap /etc/fstab
# swap was on /dev/sda5 during installation
UUID=16658fa8-8468-4cf1-955a-8bcc2e588741 none            swap    sw              0       0

cat /etc/initramfs-tools/conf.d/resume
RESUME=UUID=16658fa8-8468-4cf1-955a-8bcc2e588741

The first UUID is different from the other three. I do not know how to replace the rogue.

[p.H]

blkid shows the actual UUID. The config files just contain references which should match the actual UUID.
Update fstab and resume with the actual UUID shown by blkid.

[uthappam]

Thank you. Now both siblings are running well.

[MagicPoulp]

Why should one prefer to dual boot instead of using debootstrap to install debian 9 in a subfolder?

[p.H]

Because one wants to install Debian Buster, not 9.

[MagicPoulp]

Why is it so hard to setup a double boot 2 debian versions?

Why can't debian 10 detect and reuse the swap of debian 9 automatically?

[p.H]

Why is it so hard to setup a double boot 2 debian versions?

Can you elaborate ?

Why can't debian 10 detect and reuse the swap of debian 9 automatically?

It can, and does. But that may break the existing Debian 9 installation because it changes the swap UUID.

[MagicPoulp]

Well I have only made a double boot Windows/debian in the past. And it was much simpler. All issues were well documented somewhere, even the MSDOS bcd hack to overrid the windows efi boot to the debian one. All the rest was done just with the mouse.

Why can't a swap partition be usable and compatible for both? WHy does it override the UUID?=

[Dai_trying]

I always tell debian (and any other distros I use) not to use the swap partition, most of them will still pick up and use the partition without the need for an entry in fstab.

[Head_on_a_Stick]

most of them will still pick up and use the partition without the need for an entry in fstab

Only if the drive has a GUID partition table and the distribution is using systemd as PID1.

[Dai_trying]

most of them will still pick up and use the partition without the need for an entry in fstab

Only if the drive has a GUID partition table and the distribution is using systemd as PID1.

I guess most distros I use (or test) must be that way

[Head_on_a_Stick]

WHy does it override the UUID?=

The swap partition is reformatted (for security reasons, I think) and this changes the UUID.

You could try using the PARTUUID instead, that should remain the same even after reformatting.