I believe that the adamm.log is showing that the robotic tape library is confused and the tape library is not reporting the correct & needed information to BE.
I suggest doing a factory default of the tape library, then reset any settings needed in the library and then restart BE services.
When it was working correctly (up until 09/09/2020) with the first tape drive:
0000:0000:0002:0000 Device Name "\\.\Scsi0:"
Primary Inquiry "IBM 3573-TL 8.50"
Serial Number "IBM 3573-TL 00L4U78M0270_LL0"
Device Flags UMD, SCSI, SN(TYPE 0), SN(ELEMENT)
Device State 3, Online
Device IDs 1012, {2EECF22A-0A6E-4851-BBCD-F49CBF9A6136}
Device Name "Robotic library 0001"
Device Type 2131755008, "CHANGER FS=1"
Device Features 0x00006200: PP,RMP,RRD
1st Slot Number 1
Number Of Slots 44
Portal Slots 3
Import/Export Robotic
Drives 2
Drive Element 0 1013, "IBM ULT3580-TD4 1310204198"
Drive Element 1 1016, "MISSING DEVICE MISSING_DRV_IN_LDR_90"
starting 09/10/2020, the library was reporting incomplete info, but it was already discovered properly by BE, so it kept working:
Drive Element 0 1013, ""
Drive Element 1 1016, ""
starting on 02/18/2021, the library doesn't know what tape drives it contains:
0000:0000:0003:0000 Device Name "\\.\Scsi0:"
Primary Inquiry "IBM 3573-TL 8.50"
Serial Number "IBM 3573-TL 00L4U78M0270_LL0"
Device Flags UMD, SCSI, SN(TYPE 0)
Device State 3, Online
Device IDs 1018, {A4483B74-8D93-40CE-9CCA-5E46B990C4D5}
Device Name "Robotic library 0001"
Device Type 2131755008, "CHANGER FS=1"
Device Features 0x00006200: PP,RMP,RRD
1st Slot Number 1
Number Of Slots 44
Portal Slots 3
Import/Export Robotic
Drives 2
Drive Element 0 0, ""
Drive Element 1 0, ""
Backup Exec can see the (second) tape drive over FC, but the library isn't claiming it, so BE puts it as a stand-alone tape drive.
0000:0000:0002:0000 Device Name "\\.\Tape0"
Secondary Name "\\?\scsi#sequential&ven_ibm&prod_ult3580-td4#5&1982005&0&000200#{53f5630b-b6bf-11d0-94f2-00a0c91efb8b}"
Primary Inquiry "IBM ULT3580-TD4 97F2"
Serial Number "IBM ULT3580-TD4 1310204251"
Device Flags UMD, SCSI, SN(TYPE 0)
Device State 3, Online
Device IDs 1016, {D67DD509-0FFB-4145-BBDA-EA62D36A88EA}
Device Name "Tape drive 0001"
Device Type 134283265, "LTO 64K (64K,10,0,E,HU)"
Device Features 0x001BFA7F: PB,SFB,SRB,WFM,SFF,SRM,PEOD,EL,LU,E,SBS,DC,SDC,TA,HU,RR,W
Device Element 0, 0
Device Block Limits 512 min, 1048576 max
Device Hard Errors 0 write, 0 read
Device Soft Errors 0 write, 0 read
If you properly configure the library with a 1 tape drive partition, you will avoid the line "MISSING DEVICE MISSING_DRV_IN_LDR_90". That happens when the tape library says it has more tape drives in it than BE can find for itself.