Guys,
Just a heads up.
I've racked my head over this for days, and I've finally solved it!
If you're having issues with your micro SD external cards mounted on the N900 (especially mmcblk1p1), then despair no more.

BACKGROUND
I had issues when I bought a normal Class10 micro SD card and tried to simply use it directly from the N900. It turned out that even if the card has been previously formatted, chckdsk'd (Windows) or formatted in the N900 itself, the card would not work. By that I mean you would load it with heaps of files from Windows and suddenly the card became "read only" when tried to be used within the N900 via xTerm for example.

SOLUTION
For Windows users only (perhaps Linux users are ok?)
Download this program...
https://www.sdcard.org/downloads/formatter_4/index.html
Install Formatter and format your SD card using a standard SD card reader on your PC
It will basically re-format the partitions on the card to be just the right size as exFat (I know it won't work on N900 as exFat).
But wait there's more!
Place the card into your N900
Reformat it there.
Add a few songs/videos
xTerm and "mkdir tmp" as a test
All good!

No more READ ONLY errors!

Enjoy

UPDATE
Correction. No. The above partially worked. As soon as the card is formatted in the N900 using the standard File Manager format option, the card is formatted back to Fat32 and quick formatted incidentally. And as soon as the card fills up over 32gig, the N900 marks it as read only.
I am filling the card via Windows 8.1 and using MediaMonkey and transferring music that way.
Anyone here know how they can get past the 32gig FAT32 limit using Windows and still have the N900 'see' beyond 32gig?
So, no not solved for now unfortunately.

UPDATE 2
The card turned out to be fake! Arrghh. I ran tools as per SOSFakeFlash.com and sur enough it's a fake 64g card, with only 8gig of actual RAM!
However, I placed a real 64g SDHC card (note SDHC!), and it works fine as per the original solution above.