The Archos is a 3GB mp3 player, very small in size and lightweight.
Make sure usb mass storage module (usb_storage) and usbhid module are loaded or compiled into the kernel.
plug the xs100 via the usb cable, in a terminal type dmesg, you'll see which device was created for it, example (from dmesg output)
usb 5-2: new high speed USB device using ehci_hcd and address 7
scsi6 : SCSI emulation for USB Mass Storage devices
usb-storage: device found at 7
usb-storage: waiting for device to settle before scanning
Vendor: ARCHOS Model: GminiXS100 Rev: 0000
Type: Direct-Access ANSI SCSI revision: 04
SCSI device sdb: 5894784 512-byte hdwr sectors (3018 MB)
sdb: Write Protect is off
sdb: Mode Sense: 33 00 00 00
sdb: assuming drive cache: write through
SCSI device sdb: 5894784 512-byte hdwr sectors (3018 MB)
sdb: Write Protect is off
sdb: Mode Sense: 33 00 00 00
sdb: assuming drive cache: write through
sdb: sdb1
Attached scsi removable disk sdb at scsi6, channel 0, id 0, lun 0
Attached scsi generic sg1 at scsi6, channel 0, id 0, lun 0, type 0
usb-storage: device scan complete
usb 5-2: USB disconnect, address 7

it says the device above is sdb1,
then mount this directory with type vfat
mount -t vfat /dev/sdb1 /mnt/archos-fs

then you can copy mp3 folders etc.. into /mnt/archos-fs/Music .

to disconnect, simply umount /mnt/archos-fs . then disconnect the usb cable.

I've had not problems getting my archos rechonized by linux whatsover, neither did i have problems getting MP3s copied to the archos device.

Good luck.
Fadi.

My computer configuration:
Gentoo Linux.
2.6 based kernel.
amd64 cpu in x86_64 mode.
Nvidia fx5600 256 MB.
1 GB ram.
160 GB sata drive.