A simple and quick way to get a status of the adapters and devices in the RAID array using megacli is to run the following command:

It will output a LOT of information for the adapter, but you should see the Device Present section that shows the status of the drives. For example, the following shows a failed drive in the array:

=====
Device Present
================
Virtual Drives : 1
Degraded : 1
Offline : 0
Physical Devices : 3
Disks : 2
Critical Disks : 0
Failed Disks : 1
=====

To get more detailed information on the drives in the array, issue the command:

Which results in the following output.

=====
Adapter #0

Number of Virtual Disks: 1
Virtual Disk: 0 (target id: 0)
Name:
RAID Level: Primary-1, Secondary-0, RAID Level Qualifier-0
Size:237464MB
State: Degraded
Stripe Size: 64kB
Number Of Drives:2
Span Depth:1
Default Cache Policy: WriteBack, ReadAdaptive, Cached, No Write Cache if Bad BBU
Current Cache Policy: WriteBack, ReadAdaptive, Cached, No Write Cache if Bad BBU
Access Policy: Read/Write
Disk Cache Policy: Disk's Default
Number of Spans: 1
Span: 0 – Number of PDs: 2
PD: 0 Information
Enclosure Device ID: 252
Slot Number: 0
Device Id: 4
Sequence Number: 2
Media Error Count: 0
Other Error Count: 0
Predictive Failure Count: 0
Last Predictive Failure Event Seq Number: 0
Raw Size: 238475MB [0x1d1c5970 Sectors]
Non Coerced Size: 237963MB [0x1d0c5970 Sectors]
Coerced Size: 237464MB [0x1cfcc000 Sectors]
Firmware state: Online
SAS Address(0): 0xb221c046788723f
Connected Port Number: 0(path0)
Inquiry Data: ATA ST3250620AS K 6QE1DRKL

PD: 1 Information
=====

Notice that it shows a state of degraded. The above output is that of the degraded variety. There are two PDs (physical disks) in this span but there is only one showing in the the output.

In order to find out what drive is failed, it is actually the absence of information that shows this. Issue the following command:

This results in an output similar to the following:

=====
Adapter #0

Enclosure Device ID: 252
Slot Number: 0
Device Id: 4
Sequence Number: 2
Media Error Count: 0
Other Error Count: 0
Predictive Failure Count: 0
Last Predictive Failure Event Seq Number: 0
Raw Size: 238475MB [0x1d1c5970 Sectors]
Non Coerced Size: 237963MB [0x1d0c5970 Sectors]
Coerced Size: 237464MB [0x1cfcc000 Sectors]
Firmware state: Online
SAS Address(0): 0xb221c046788723f
Connected Port Number: 0(path0)
Inquiry Data: ATA ST3250620AS K 6QE1DRKL
=====

In the case of the above, there are two drives, one each in slot 0 and slot 1. Since slot 1 is not listed above, the assumption is that the bad drive is slot 1. Slot 0 is showing with no errors. Note that the command

also shows the missing slot 1 information, indicating that slot 1 is the failed drive.