Choosing whether to upgrade Mandriva or not is up to you. I'll assume that if you don't upgrade, there's a reason for it (older hardware, too much of a PITA, can't afford the downtime, etc.). What I planned to suggest works regardless.
Mounting and unmounting is pretty simple. When you plug a device into the machine, if all goes well, the kernel creates a device file. The device file is located in /dev and for a USB storage device (thumb drive, external HD, whatever), it's usually named sdX? (where X is a letter from a-z and ? is a number starting from 1). So, an example would be "/dev/sdc1."
For what you want to do, you cannot
read/write files directly to the device file. You must mount it first. Mounting a device just integrates it into the filesystem. What you were doing originally would have worked as you expected had your USB device been mounted at /media/sda.
Enough talk... a practical example. There are other ways of doing this (see jlinkels' reply using df and Larry Webb's reply using /proc/partitions). Open a terminal, command line, shell, or whatever you like to call it. Plug in your USB drive. Now type:
You'll get a whole bunch of info, but you're only interested in a small portion--should be the last bit. I just plugged in one of my own, and this is the tail end of what I got:
[964443.750096] usb 1-1: new high speed USB device using ehci_hcd and address 26
[964443.905301] usb 1-1: configuration #1 chosen from 1 choice
[964443.906204] scsi23 : SCSI emulation for USB Mass Storage devices
[964443.906400] usb-storage: device found at 26
[964443.906405] usb-storage: waiting for device to settle before scanning
[964448.900376] usb-storage: device scan complete
[964448.900974] scsi 23:0:0:0: Direct-Access LEXAR JUMPDRIVE ELITE 2000 PQ: 0 ANSI: 0 CCS
[964448.901929] sd 23:0:0:0: Attached scsi generic sg6 type 0
[964448.904498] sd 23:0:0:0: [sdg] 502880 512-byte logical blocks: (257 MB/245 MiB)
[964448.905299] sd 23:0:0:0: [sdg] Write Protect is off
[964448.905307] sd 23:0:0:0: [sdg] Mode Sense: 43 00 00 00
[964448.905313] sd 23:0:0:0: [sdg] Assuming drive cache: write through
[964448.908563] sd 23:0:0:0: [sdg] Assuming drive cache: write through
[964448.908675] sdg: sdg1
[964448.960796] sd 23:0:0:0: [sdg] Assuming drive cache: write through
[964448.960907] sd 23:0:0:0: [sdg] Attached SCSI removable disk
The important stuff is highlighted in red. For me, the USB thumbdrive has been assigned two device files: /dev/sdg and /dev/sdg1. We're not concerned with the /dev/sdg device file--only /dev/sdg1 (the number represents a partition on the device--generally only one partition on a thumb drive). So now you mount it:
sudo mount -o uid=xxx,gid=yyy /dev/sdg1 /media/usb_drive
A couple things:
(1) You will need root privileges to execute mount. On Ubuntu systems, we use sudo. I'm not familiar with Mandriva. You may need to execute "su" first and enter the root password--then execute the mount command without the sudo at the beginning.
(2) Replace the "xxx" and "yyy" parts of the command with your text username and text groupname. For example, if your username is beeson76, do something like "uid=beeson76,gid=beeson76". Watch the spacing and if you don't know your default group, you can probably leave off the gid portion altogether. This option will allow your regular account to save your backup to the drive--no special permissions needed.
(3) Replace "/media/usb_drive" with whatever directory you want. It can be anything so long as the directory exists. After the mount is successful, writes to that directory will be diverted to your USB drive. Then update your command/script so that the backup is written to the same directory that you mounted the USB drive to.
When you want to remove the USB thumbdrive, execute:
sudo umount /dev/sdg1
Again, the sudo is only to get root privileges. Replace that with whatever Mandriva does by default.
Now, with all that said, there are some things that can automate the process a little bit (e.g. writing one or two udev rules to automatically mount one or more USB thumb drives to the appropriate location). If you're interested, either I or some other folks here can help you with that later. I figure this is enough to chew on at the moment.