Please let me know which camera you use.

some of camera using SxS memory card format it as UDF file system.

and NVS2501 doesn't support it.

Depends on Camera you use,

i.e.
XDCAM EX1/3 or Canon C300 formats SxS in FAT, compatible with all NVS models except NVS-1501 Please note, It's not recommended to use NVS2525A UDF Special with non-UDF formatted cards.

PMW-200 or 500, Arri Alexa format SxS card in UDF, compatible with NVS-2825 (or NVS-2525A UDF Special) only

PMW-5 or 55 format SxS card in exFAT, compatible with NVS-2825 only

P2HD formats P2 cards in FAT, compatible with NVS-2825/2525/2501/2500