Why is my Avic-F900BT not detecting my Sandisk 4 Gig Jump Drive?

I had no luck with my 4gb kingston either, they only support 2gb max.

try reformatting as FAT instead of FAT32

Also, if your sandisk flash drive is running U3 as so many of them are these days, try using a U3 removal tool if you don't use all that U3 stuff and see if that helps the drive get picked up. U3 causes the flash drive to show up as both a CD-ROM drive and a flash drive, this might cause detection issues I'd suppose if the HU's only looking for a usb mass storage device and sees a random CD-ROM. I ran into a similar issue the first time I tried making a U3 drive bootable to a linux install.

I have a Kingston 4 and 8 gig thumb drive- both work fine. The trick is to plug them in only AFTER the system has fully started. Also, I found that if I turn it off without removing the drive, it takes twice as long to start up than if I remove the thumb drive and then turn off. It's hard to remember to unplug it everytime I get out of the car- but worth having 8 gigs of music!