Mint USB flash-drive option auto-mount is ON.
Selecting unmount or eject produces a message:
"There is data to be written to xxx before it can be dis-mounted"
The USB busy-lamp flashes constantly, for what reason ?
The USB disks are new, pre-formatted as FAT32, and empty !

I tested other old flash disks with diff formats, same problem.

Last test- Mounted USB SATA HD 4GB with 6 partitions.
The select-lamp is ON but not flashing, I take that as 'idle'.
Unmount or Eject simply turns the lamp off.

Sanity test- Same flash-drives in two old laptops,
LinuxLite & WattOS, both unmount as normal.

Any and all help is appreciated.

Check iotop and maybe lsof to see what's happening.

1 members found this post helpful.

There is nothing to help with. Unmounting typically triggers a sync to flush the buffer before ejecting. And given that they're FAT32, they probably have some sort of Windows junk on them (you don't mention brand, but several models have 'convinient drivers' and other software in a non-erasable partition). So when you mount them, they open things...which have to be closed.

Also, if you tried different drives/formats, you could very well have file-indexing turned on, which is trying to index that mounted drive (since you auto-mounted it). That process has to stop, so that means there is data in process going to/from that drive...leading to a message.

The fact that the older laptops don't do it is a good indication of this; indexing and other such things weren't common in several 'light' distros.

1 members found this post helpful.

I have retro-graded back to release 20. Problems are gone.

I have the same problem in Linux Mint 21. It never happens in Linux 20 for me either.

Recently completed a "distro-dance" of distrowatch's 64bit top dozen. They ALL have this 'ailment',,, some worse than others.
I found Manjaro seems to handle this better,,, I could mount/search/dismount without messages.
Writing on the USB, will cause a 'safe dismount' message to appear in the file manager.
However,,, writing a 1GB++ amount will cause Manjaro to pause writing, do something else???, then resume writing.
I suppose its flushing a buffer on the hard-drive. My machine does not have activity lights, its kinda boring, no messages !