NDF = SQL Server Secondary Data File
for backup information, check http://help.sap.com/saphelp_nw04s/helpdata/en/f2/31add4810c11d288ec0000e8200722/content.htm
NDFs are not used to span multiple disks at all.
NDF is the extension for secondary data-files used to make up a SQL DB. MDF being the primary and LDF being logs.
SAP by default (on SQL) will use an MDF and two NDF files - however you can have as many NDFs as you need/want. (ie you could have 5 mirror sets for the data and put each file on its own mirror set - yes, its on a separate volume set, but is so by design, not due to the disk layout! Or as is common on SAP you would have one data area on the San and locate all the data files on the same volume set.)
To backup SAP correctly you also need to backup the Master DB (this contains info about the databases and where they live) MSDB, and the actual DB.
Recovery is simple. you can either create the DB layout prior to restore ie the M N and LDF files at the size needed and cat the tape, then restore. Else you can do it all from the tape and have the restore create them for you - either way the files need to be created at the correct size before the data is sucked back into the DB. with either If you are doing a PIT recover you will add the logs back on over the restored DB leaving the DB offline and ready for additional logs except for the last in the series where you'd bring it back on line.
PS data-files are not recommended for use on Raid 1 sets, it was an example only. Data should be on Raid 5 or raid 01 (10 - strip set mirrored).
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