Use MBR partitions only.
Multiman is not the best choice for managing files on a ntfs partition. Its file manager uses a read-only driver (PFS) & certain USB drives aren't recognised properly (in which case you must use a USB.CFG file with the device's ID).
Even with a proper USB.CFG, some devices/partitions cannot be read by the mm file manager, I have never bothered looking for an explanation.
Anyway, generally speaking for ntfs, you are better off using Irisman or Managunz.
Regarding webMAN-MOD, sMan & other sprx based homebrew, there is an additional problem. The ntfs library used in those projects uses a cap on the maximum supported number of ntfs clusters for a single partition. When using big partitions (>1Tb), the default cluster size used by partitioning utilities usually create too many clusters for the ntfs library. The bigger the partition, the more likely the issue. There is an easy workaround though, you can increase the cluster size, any decent tool can do that on an existing partition.