Both Linux and MacOS can handle NTFS so if you are needing to store
files over 4Gb that's worth considering, but as you say - good old
FAT32 is still the one that is 'one size fits all' :-)

BTW - did you really mean SSD ( Solid State Drive )? Using one of
these as external storage is a terrible waste of money - SSD's are
much better as boot drives where their superior speed really makes a
difference !

Thanks for the info about NTFS. I'm kind of new to Linux, and did
not know that.

I asked the question with reference to a USB Ironkey drive, which is
a very secure (encrypted) solid-state plug-in I bought for some special
files I don't want falling into the wrong hands. But since I find it
necessary for now to use Ubuntu, Mac OS X, and Windows systems, I need
to be able to access that drive fron any one of those systems nearly
every day. The Ironkey came with NTFS formatting, so I guess I can
just leave it that way. Thanks again.

USB drives are usually called Flash Drives or Thumb Drives so that
caused the confusion. SSD is an internal HD replacement that is solid
state and a totally different animal

Regarding your Iron Key drive, looks like you are good to go on all
counts here ... https://www.ironkey.com/hardware-encryption :-)

NTFS is more wide-spread and usable