As per advice from MSFN/Neowin, the following comman did the trick!

I have external disks (many of them) formatted as exFAT, and if they are dirty, Windows mounts them as Read-Only without informing me of this in any way shape or form. Closing all programs that are using the volume, and then running chkdsk /f x: (where x is your drive letter) fixes this in that particular scenario.

Microsoft Windows XP [Version 5.1.2600]
© Copyright 1985-2001 Microsoft Corp.

D:\Users\Administrator>chkdsk /f E:
The type of the file system is exFAT.

Chkdsk cannot run because the volume is in use by another
process. Chkdsk may run if this volume is dismounted first.
ALL OPENED HANDLES TO THIS VOLUME WOULD THEN BE INVALID.
Would you like to force a dismount on this volume? (Y/N) y
Volume dismounted. All opened handles to this volume are now invalid.
Volume Serial Number is 1206-4681
Windows is verifying files and folders...
Volume label is DATA.
Corruption was found while examining files in directory \Drivers\Vostro 1510\Arc
hive\Diagnostics\ (0).
Corruption was found while examining files in directory \Drivers\Vostro 1510\Arc
hive\Diagnostics\ (1).
Corruption was found while examining files in directory \Drivers\Vostro 1510\Arc
hive\Diagnostics\ (2).
Corruption was found while examining files and directories.
Corruption was found while examining the volume bitmap.
File and folder verification is complete.
Windows has made corrections to the file system.

823144448 KB total disk space.
328008960 KB in 68106 files.
1144064 KB in 4469 indexes.
0 KB in bad sectors.
768 KB in use by the system.
493990656 KB available on disk.

262144 bytes in each allocation unit.
3215408 total allocation units on disk.
1929651 allocation units available on disk.

D:\Users\Administrator>

Thanks for all the replies guys.