Solution Title: How to change the block size in Windows NT/2000 Solution ID: legato7083 
Here is the solution: INITIAL CHECKS:

First, check the block size the tape was written with:
- Load the tape with no mount to one of the drives
- Scan the tape with verbose switch on
- you can read from the scanner output (Sample below: shows "record size 655536") when you devise the number shown by 1024 this will give you the block size of the tape (ie. 65536/1024=64kb)

Sample:
C:\>nsrjb -lnv -f \\.\Tape0 -S Slot_Number
C:\> scanner -vvv \\.\Tape0
scanner: Opened \\.\Tape0 for read
scanner: Rewinding...
scanner: Rewinding done
scanner: Reading the label...
scanner: Reading the label done
scanner: scanning dlt7000 tape Full.001 on \\.\Tape0
scanner: volume id 128186625 record size 65536
created 11/30/01 10:21:29 expires 11/30/03 10:21:29
...

CHANGING THE BLOCK SIZE:

1. Run REGEDIT (the Registry Editor)
2. For non-Ultra2 host adapters, open:
\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\aic78xx

Note: You can also get the driver information in Control Panel.

3. Create a key named Parameters (if it doesn't exist) by selecting from the menu:
Edit > New > Key and entering Parameters for Key Name.

4. Open the Parameters key and create a key named Device by selecting from the menu:
Edit > New > Key and entering Device for Key Name.

5. Open the Device key and create a DWORD named MaximumSGList by selecting from the menu:
Edit > New > DWORD and entering MaximumSGList for DWORD Name.

6. Open the MaximumSGList Value Name and replace the existing Value Data in Decimal with one of the values from (Table 6)

Table 6. Block Sizes
Block Size Decimal Value Hex Value
64k 17 decimal 11 hex
96k 25 decimal 19 hex
128k 33 decimal 21 hex
256k 65 decimal 41 hex
512k 129 decimal 81 hex

7. Specify the size of the writing block size:
By setting the environment variable of the default block size used by NetWorker to a value between the default and the maximum block size (Sample 8.2 below)
For a list of block size for different devices call Techinical Support or go to legato10382
This can be done by openning "System" from "Control Panel" and add the env. var. in the "System Variables" portion under the "Advanced" Tab.
NetWorker will use this env. var. value to write to tapes.

Sample 7.1
NSR_DEV_BLOCK_SIZE_DLT7000=64
in this case the tapes are going to be written at 64KB and the system can read any tape written in these values (64, 96 and 128KB)

8. After rebooting the machine
Run the inquire command to locate your drives, and run the mt command to check if the maximum blocksize has changed.

Note: in Sample 8.2, the block size has been changed to 128Kb, this works out
by dividing 131072 by 1024, and the same for (65536/1024=64Kb)

Sample 8.1
in this example the key is:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\aic78xx\Parameters\Device
\_ MaximumSGList = 33 decimal

C:\>inquire
scsidev@0.5.0:NEC CD-ROM DRIVE:4661.06|CD-ROM
scsidev@0.6.0:ARCHIVE Python 06408-XXX8071|Tape, \\.\Tape0
scsidev@1.0.0:HP C5173-7000 3.02|Autochanger (Jukebox)
scsidev@1.2.0:QUANTUM DLT7000 1732|Tape, \\.\Tape1
scsidev@1.3.0:QUANTUM DLT7000 1732|Tape, \\.\Tape2
scsidev@2.0.0:SEAGATE ST39204LC 0002|Disk, \\.\PHYSICALDRIVE0
scsidev@2.1.0:QUANTUM ATLAS V 9 SCA 0201|Disk, \\.\PHYSICALDRIVE1
scsidev@2.6.0:DELL 1x4 U2W SCSI BP 5.35|Processor

Sample 8.2
C:\>mt -f \\.\Tape1 status
\\.\Tape1:
Media Capacity = 15.20GByte
Media Remaining = 13.60GByte
Media Blocksize = 0
Media Partition Count = 0
Media is not write protected
default blocksize = 65536
maximum blocksize = 131072
minimum blocksize = 1
MaximumPartitionCount = 0
Partition = 0
Logical block position = 44815
EOTWarningZoneSize = 0
CompressionEnabled
Features: ... 
Here is the problem or goal: Volume "XXX" on device "XXX": Block size is xxxxx bytes not xxxxx bytes. Error: 'Unkown error 234' How to change the block size in Windows NT/2000 How to change device block size on NT Block size environment variables settings