@crush Yes I did that, didn't realise I had to do it twice. All sorted now :-)

@solidhyunkel Hello and welcome to this forum.

See Running ROMs from a USB drive for detailed instructions about that.

https://retropie.org.uk/docs/Running-ROMs-from-a-Network-Share/

It can be done if you split out different romsets across different drives and arrange the mount points manually.
So for example I have roms on a 16gb usb and disc images (psx, segacd) and bios on a 64gb drive.
Here is an example of the set up in /etc/fstab:

LABEL=TOSHIBA16 /mnt/toshiba16 vfat defaults,noatime,umask=0000,uid=1000,gid=1000,nofail,x-systemd.device-timeout=15 0 0
/mnt/toshiba16/roms /home/pi/RetroPie/roms none bind,nofail,x-systemd.device-timeout=15 0 0
LABEL=TOSHIBA64 /mnt/toshiba64 ext4 defaults,noatime,nofail,x-systemd.device-timeout=15 0 2
/mnt/toshiba64/roms/psx /home/pi/RetroPie/roms/psx none bind,nofail,x-systemd.device-timeout=15 0 0
/mnt/toshiba64/roms/segacd /home/pi/RetroPie/roms/segacd none bind,nofail,x-systemd.device-timeout=15 0 0
/mnt/toshiba64/BIOS /home/pi/RetroPie/BIOS none bind,nofail,x-systemd.device-timeout=15 0 0

@BillyH You can also press F4 and type the following command: df -h /
This will show you how big the sd card is and how much space is left.

Also if your Pi is conntected to Wifi, then you can open Windows explorer on your computer and type \\RETROPIE in the top bar to see what is inside the roms and bios folders (and make changes).